Optical Fiber Preform Thermal Diffusion Control
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Solution Overview
Problem
The generation of foreign substances on the inner surface of glass pipes during thermal diffusion of alkali metal elements leads to crystallization, which can cause cracks and increase attenuation and decrease the strength of optical fibers.
Innovation Solution
A method involving controlled thermal diffusion of alkali metal salts, where the vapor pressure is maintained at 0.1 kPa or less initially and then increased, with a controlled temperature increase rate of 5 °C/min to 15 °C/min, to prevent excessive accumulation and crystallization, using alkali metal salts like potassium bromide or potassium iodide, and subsequent etching to remove impurities.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If alkali metal salt is heated to high temperature to generate sufficient vapor pressure for thermal diffusion, then the diffusion efficiency of alkali metal into glass pipe is improved, but foreign substances are generated on the inner surface of the glass pipe
Solution Approach 1:
The patent applies preliminary action by first heating the alkali metal salt to a lower temperature (maintaining vapor pressure at 0.1 kPa or less) before the main thermal diffusion process. This preliminary heating stage prevents excessive vapor generation and foreign substance formation on the glass pipe inner surface, while still allowing controlled diffusion to occur during the subsequent higher temperature stage.
Solution Approach 2:
The patent changes the temperature parameter dynamically during the thermal diffusion process. It specifies heating the alkali metal salt at a controlled temperature to maintain vapor pressure at 0.1 kPa or less initially, then subsequently increasing the temperature. This parameter change approach optimizes diffusion efficiency while minimizing foreign substance generation on the glass pipe surface.
2Quantity of substance
If the vapor pressure of alkali metal salt is increased to enhance diffusion, then the concentration of alkali metal in glass pipe is improved, but crystallization occurs on the inner surface
Solution Approach 1:
The patent applies preliminary action by controlling the vapor pressure at 0.1 kPa or less during the initial heating stage. This prevents excessive alkali metal accumulation on the glass pipe inner surface that would lead to crystallization, while still allowing gradual diffusion to occur. The controlled low-pressure stage prepares the system for subsequent higher concentration diffusion without causing instability.
Solution Approach 2:
The patent changes the vapor pressure parameter in stages, initially maintaining it at 0.1 kPa or less, then subsequently increasing it. This staged parameter change allows the glass pipe to accommodate alkali metal diffusion progressively, preventing sudden concentration spikes that would cause crystallization while still achieving the desired final concentration.
3Reliability
If thermal diffusion is performed at high temperature to achieve sufficient alkali metal concentration, then the optical fiber attenuation is reduced, but the glass pipe inner surface quality deteriorates
Solution Approach 1:
The patent applies preliminary action by performing initial heating at controlled temperature with vapor pressure at 0.1 kPa or less before the main high-temperature diffusion process. This preliminary stage prevents foreign substance formation and maintains inner surface quality, while the subsequent high-temperature stage achieves the necessary alkali metal concentration for low attenuation optical fibers.
Solution Approach 2:
The patent changes the temperature and vapor pressure parameters in a staged manner. Initially, parameters are controlled to maintain vapor pressure at 0.1 kPa or less to protect surface quality. Then parameters are increased to achieve high-temperature diffusion for sufficient alkali metal concentration, ensuring both surface quality and optical performance.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This method effectively suppresses the generation of foreign substances, preventing crystallization and enabling the production of high-quality optical fibers with low attenuation and improved strength.
Implementation Method 1
a thermal diffusion process for oxidizing and thermally diffusing an alkali metal element into the inner side of a silica glass pipe by heating the glass pipe
Implementation Method 2
the alkali metal salt is heated at a temperature for making the vapor pressure of the alkali metal salt to be 0.1 kPa or less
Implementation Method 3
the alkali metal element is oxidized so as to thermally diffuse into the inner side of the glass pipe
Implementation Method 4
heating the glass pipe by means of a heat source performing relative movement in the longitudinal direction of the glass pipe
Implementation Method 5
a collapsing process for forming a core rod by collapsing the glass pipe after the thermal diffusion process
Data Source
AI summary
A method for making a high quality optical fiber preform, wherein the generation of foreign substance on the inner surface of a glass pipe can be suppressed at the process of carrying out thermal diffusion of an alkali metal element. The method comprises: (1) a thermal diffusion process for thermally defusing the alkali metal element into the inner side of a silica glass pipe by heating the glass pipe from the outside by a heat source while vapors of alkali metal salt generated by heating an alkali metal salt is supplied to the inside of the glass pipe from an end thereof; (2) a collapsing process for forming a core rod by collapsing the glass pipe after the thermal diffusion process; and (3) a process for adding a cladding part around the circumference of the core rod, wherein at the start of the thermal diffusion, the alkali metal salt is heated at a temperature for making the vapor pressure of the alkali metal salt to be 0.1 kPa or less, and thereafter the alkali metal salt is heated up to a temperature for making the vapor pressure of the alkali metal salt to be larger than 0.1 kPa.